This invention relates to a receiver for receiving a digital signal transmitted through a transmission path to reproduce the transmitted digital signal as a reproduced digital signal.
A receiver of the type described is supplied with a transmitted digital signal which is transmitted through a transmission path of a first frequency characteristic and which is inevitably subjected to noise. It is therefore necessary in the receiver not only to equalize the transmitted digital signal but also to remove the noise. To this end, the receiver comprises a feedback digital filter for equalizing the transmitted digital signal. In addition, a proposal is made about reduction of the noise by using a matched filter which has an infinite delay time. However, it is practically difficult to implement such a matched filter having the infinite delaying time.
In order to practically reduce the noise, an improved receiver is disclosed in an article contributed by J. Salz to The Bell System Technical Journal Vol. 52, No. 8 (October 1973), pages 1341 to 1373, under the title of "Optimum Mean-Square Decision Feedback Equalization". The improved receiver comprises a linear filter and the feedback digital filter. The linear filter is used as a filter section and has a second frequency characteristic. The feedback digital filter is used as an equalizer section. The linear filter filters the transmitted digital signal into a filtered digital signal. The feedback decision equalizer equalizes the filtered digital signal into an equalized digital signal in response to controllable tap gains. The controllable tap gains are controlled by a tap gain controller in accordance with the first and second frequency characteristics to make the linear filter produce the equalized digital signal as the reproduced digital signal.
However, the linear filter can not sufficiently get rid of the noise introduced into the transmitted digital signal. Additionally, the filtered digital signal is accompanied with a noise signal and is applied to the feedback digital filter together with the noise signal. The equalized digital signal is produced with the noise from the feedback decision equalizer. This makes it difficult to achieve a satisfactory signal-to-noise ratio in the reproduced digital signal.